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United States Patent |
5,550,367
|
Plesko
|
August 27, 1996
|
System for extending the operating range of a beam scanner
Abstract
Non-Imaging beam conditioning optical elements are disclosed which allow
nearly full power usage from a light source, in a beam scanning
information readout device such as a bar code reader. Light which would
have been otherwise wasted by using stop apertures as in prior art
scanning devices, is utilized efficiently by apportioning a greater amount
of the light to the far out targets and less to the close in targets,
effectively solving the problem of signal amplitude variation over a long
working range. At the same time the novel optical elements provide
multiple focal ranges which may be made to overlap thereby providing small
beam cross sections for near and distant targets, significantly extending
the depth of scanning range over prior art scanners. Amplification and
signal processing of received light signals is simplified thereby
eliminating the need for complex and fussy AGC amplifiers and limiters to
deal with both strong and weak signals from close and distant targets
respectively. Novel signal processing circuitry is also disclosed for
adaptively processing received signals and for reducing variation
associated with laser diodes, optical elements, and target related
variables such as such as distance, reflectivity and contrast.
Inventors:
|
Plesko; George A. (Media, PA)
|
Assignee:
|
GAP Technologies, Inc. (Media, PA)
|
Appl. No.:
|
229728 |
Filed:
|
April 19, 1994 |
Current U.S. Class: |
235/462.22; 235/455; 235/462.26 |
Intern'l Class: |
G06K 007/10 |
Field of Search: |
235/455,467,472
359/615,558
|
References Cited
U.S. Patent Documents
3860794 | Jan., 1975 | Knockeart | 235/455.
|
4575625 | Mar., 1986 | Knowles | 235/467.
|
4639589 | Jan., 1987 | Weber et al. | 250/203.
|
4818886 | Apr., 1989 | Drucker | 235/462.
|
4871904 | Oct., 1989 | Metlitsky et al. | 235/467.
|
4877949 | Oct., 1989 | Danielson | 235/462.
|
4958064 | Sep., 1990 | Kirkpatrick | 235/384.
|
4983817 | Jan., 1991 | Dolash | 235/462.
|
4999491 | Mar., 1991 | Semler et al. | 250/236.
|
5059779 | Oct., 1991 | Krichever et al. | 235/467.
|
5140141 | Aug., 1992 | Inagaki et al. | 235/462.
|
5144120 | Sep., 1992 | Krichever et al. | 235/472.
|
5149949 | Sep., 1992 | Wike, Jr. | 235/467.
|
5168149 | Dec., 1992 | Dvorkis et al. | 235/472.
|
5170277 | Dec., 1992 | Bard et al. | 359/210.
|
5187612 | Feb., 1993 | Plesko | 359/896.
|
5438187 | Aug., 1995 | Reddersen | 235/462.
|
Foreign Patent Documents |
63-263585 | Oct., 1988 | JP.
| |
Primary Examiner: Hajec; Donald T.
Assistant Examiner: Frech; Karl D.
Attorney, Agent or Firm: Reed Smith Shaw & McClay
Parent Case Text
This is a continuation-in-part of U.S. patent application Ser. No.
07/776,663, now U.S. Pat. No. 5,371,347 entitled "Electro-Optical Scanning
System With Gyrating Scan Head", filed Oct. 15, 1991.
Claims
What is claimed is:
1. An optical system for distinguishing features of a target in a working
distance range comprising:
a light producing means for generating a first group of light rays and a
second group of light rays from a light source position;
a focus means for converging said first group of light rays and said second
group of light rays in said working distance range;
an optic means disposed between said source and said focus means, said
optic means having a thickness that is less fit an a length between first
and second ends of said working distance range;
said optic means having at least first and second optical path regions for
simultaneously receiving said first group of light rays and said second
group of light rays, respectively, from said source position and for
refractively forming, with said focus means, a third group of light rays
and a fourth group of light rays, respectively; and
wherein said third group of light rays converge in said first end of said
working distance range and said fourth group of light rays converge in
said second end of said working distance range, said second end of said
working distance range being downrange from said first end of said working
distance range, and wherein said first optical path region is formed of an
opening in said optic means.
2. An optical system according to claim 1 wherein, said second optical path
region of said optic means consists of a refractive material having an
index of refraction greater than one.
3. An optical system according to claim 2 wherein, said optic means has at
least one step.
4. An optical system according to claim 3 wherein, said focus means and
said optic means are integrally combined as a single element.
5. An optical system according to claim 2 wherein, said optic means is
generally flat with at least one opening.
6. An optical system according to claim 5 wherein, said system is adapted
for use in a bar code scanner.
7. An optical system according to claim 5 wherein, said opening is a
circular opening.
8. An optical system according to claim 7 wherein, a first portion of the
light beam generally converges in a first focal range and a second portion
of the light beam generally converges in a second focal range farther down
range than said first focal range, said first and second focal ranges
being within said working distance range.
9. An optical system according to claim 5 wherein, said opening is
generally slot shaped.
10. An optical system according to claim 5 wherein, said optic means
includes a bar shaped area for light to pass through.
11. An optical system according to claim 1 wherein, said optic means is
adapted to apportion a relatively smaller amount of light to a close-in
working distance range and a relatively larger amount of light to a
far-out working distance range.
12. An optical system according to claim 11 wherein, said system is adapted
for use in a bar code scanner.
13. An optical system according to claim 1 which further includes;
a scanning means for scanning said light beam across a target to read
information therefrom;
a photo electric converter for receiving light from a target and producing
electrical signals which are responsive to received light; and
a circuit means for setting a first trigger threshold at a predetermined
proportion of the peak of said electrical signals and a second trigger
threshold at a predetermined minimum level of said electrical signals
comprising:
a detector means for determining said signal peak; and
a trigger means for switching an output to a first state when said signal
level reaches said first trigger threshold and to a second state when said
signal level falls to said second trigger threshold.
14. An optical system according to claim 13 wherein, said system is adapted
for use in a bar code scanner.
15. An optical system according to claim 1 wherein, said optic means
includes a surface for effecting a continuum of focal zones.
16. An optical system according to claim 1 wherein, said optic means
comprises a holographic optical element.
17. An optical system according to claim 1 wherein, said optic means is a
reflective optic means.
18. An optical system according to claim 17 wherein, said optic means
produces a plurality of light source points by multiple reflections
therein.
19. An optical system according to claim 1 which further includes;
a scanning means for scanning said light beam across a target to read
information therefrom;
a photo electric converter for receiving light from a target and producing
electrical signals which are responsive to received light; and
a circuit means for controlling the light output of said light producing
means in response to the peak level of said received electrical signals
comprising:
a detector means for determining said peak signal level; and
a feed back control means for adjustably varying the light output of said
light producing means.
20. An optical system according to claim 19 wherein, said system is adapted
for use in a bar code scanner.
21. An optical system according to claim 1, wherein said first and second
different optical path regions have different indexes of refraction.
22. An optical system according to claim 1, wherein said source position of
said light source corresponds to said first source point.
23. An electro-optical system for distinguishing features of a target in a
working distance range comprising:
a light source for projecting a light beam on a target to be read;
a photo electric converter for receiving light from said target and
producing electrical signals which are responsive to received light; and
an adaptive circuit means for setting a first trigger threshold at a
predetermined proportion of the peak of said electrical signals and a
second trigger threshold at a predetermined minimum level of said
electrical signals comprising:
a detector means for determining said signal peak; and
a trigger means for switching an output to a first state when said signal
level reaches said first trigger threshold and to a second state when said
signal level falls to said second trigger threshold.
24. An electro-optic system according to claim 23 wherein said electrical
signals are differentiated signals.
25. An electro-optic system according to claim 24 wherein, said trigger
means includes a pseudo-power supply voltage level wherein said voltage
level is proportionally related to a peak level of said signal level.
26. An electro-optic system according to claim 25 wherein said trigger
means is a CMOS trigger means.
27. An electro-optic system for distinguishing features of a target in a
working distance range comprising:
a light source for producing a light beam;
a scanning means for scanning said light beam across a target to read
information therefrom;
a photo electric converter for receiving light from a target and producing
electrical signals which are responsive to received light;
a circuit means for controlling the light output of said light producing
means in response to the peak level of said received electrical signals
comprising:
a detector means for determining the peak level of said signals; and
a feed back control means for adjustably varying the light output of said
light producing means.
28. An electro-optic system according to claim 27 wherein said light output
is relatively less for close in targets and more for far out targets.
29. An electro-optical system according to claim 27 wherein, said system is
adapted for use in a bar code scanner.
30. An optical system for distinguishing features of a target in a working
distance range comprising:
a light producing means for generating a light beam from a light source at
a source position;
a focus means for converging said light beam in said working distance
range; and
a transparent optic means having first and second different optical path
regions for simultaneously receiving light from said source, said first
and second optical path regions of said transparent optic means being
disposed proximate said focus means;
wherein said first optical path region effects at least one focal range in
said working distance range and wherein said second optical path region of
said transparent optic means is formed of a translucent means for
scattering an unwanted portion of said light beam.
31. The optical system of claim 30, wherein said first optical path region
is formed of an opening in said transparent optic means.
32. A method for distinguishing features of a target in a working distance
range comprising the steps:
(A) generating a first group of light rays and a second group of light rays
from a light source at a source position;
(B) simultaneously receiving said first group of light rays and said second
group of light rays, respectively, from said source position with at least
first and second different optical path regions of optic means between
said source and a focus means, and refractively forming, with said at
least first and second different optical path regions, respectively, a
third group of light rays and a fourth group of light rays; and
(C) converging, with said focus means, said third group of light rays in a
first end of said working distance range and said fourth group of light
rays in a second end of said working distance range, said second end of
said working distance range being downrange from said first end of said
working distance range;
wherein said optic means has a thickness that is less than a length between
said first and second ends of said working distance range, and said first
optical path region is formed of an opening in said optic means.
33. A method according to claim 32 wherein, said second optical path region
of said optic means consists of a refractive material with an index of
refraction greater than one.
34. A method according to claim 33 wherein, said optic means has at least
one step.
35. A method according to claim 34 wherein, said focus means and said optic
means are integrally combined as a single element.
36. A method according to claim 33 wherein, said optic means is generally
flat with at least one opening.
37. A method according to claim 36 wherein, said opening is a circular
opening.
38. A method according to claim 37 wherein, step (c) further comprises
converging a first portion of the light beam in a first focal range and a
second portion of the light beam in a second focal range farther down
range than said first focal range.
39. A method according to claim 36 wherein, said opening is generally slot
shaped.
40. A method according to claim 36 wherein, said optic means includes a bar
shaped area for light to pass through.
41. A method according to claim 32 wherein, step (c) further comprises
apportioning a relatively smaller amount of light to a close-in working
distance range and a relatively larger amount of light to a far-out
working distance range.
42. A method according to claim 32, further comprising the steps of:
(D) scanning said light beam across a target to read information therefrom;
(E) receiving light from said target and producing electrical signals which
are responsive to received light; and
(F) setting a first trigger threshold at a predetermined proportion of the
peak of said electrical signals and a second trigger threshold at a
predetermined minimum level of said electrical signals.
43. A method according to claim 32 wherein, said optic means effects a
continuum of focal zones.
44. A method according to claim 32 wherein, said optic means comprises a
holographic optical element.
45. A method according to claim 32 wherein, said optic means is a
reflective optic means.
46. A method according to claim 45 wherein, said optic means effects a
plurality of light source points by multiple reflections therein.
47. A method according to claim 32, further comprising the steps of:
(D) scanning said light beam across a target to read information therefrom;
(E) receiving light from said target and producing electrical signals which
are responsive to received light; and
(F) controlling, with a circuit means, a light output of a light producing
means in response to a peak level of said electrical signals, wherein said
circuit means is formed of a detector means for determining said peak
signal level and a feed back control means for adjustably varying the
light output of said light producing means.
48. A method according to claim 32, wherein said first and second different
optical path regions have different indexes of refraction.
49. A method according to claim 32, wherein said source position of said
light source corresponds to said first source point.
50. A method for distinguishing features of a target in a working distance
range comprising the steps of:
(A) generating a light beam from a light source at a source position;
(B) simultaneously receiving light from said source position with at least
first and second different optical path regions of a transparent optic
means disposed proximate a focus means;
(C) converging, with said focus means, said light beam in said working
distance range;
(D) effecting, with said first optical path region of said transparent
optic means, at least one focal range in said working distance range; and
(E) scattering, with said second optical path region of said transparent
optic means, an unwanted portion of said light beam.
Description
BACK GROUND OF THE INVENTION
In a laser beam scanner a small spot of light is directed toward a target
and is swept rapidly across the target. After reflection from the target a
photo electric converter such as a photo diode detects the reflected light
and converts it to electronic signals representing features of the target.
To successfully resolve features of the target it is necessary that the
spot size be about the size of, or smaller than, the smallest features of
the target.
A bar code reader is an important commercial application for beam scanners
and is referred to herein as a typical example of a specific application
for the present invention.
In typical barcode scanners imaging optical components such as converging
lenses and stop apertures are placed in front of a light source such as a
laser diode causing the beam to converge to a small spot about several
inches away from the source. The densest bar codes, i.e. those with the
narrowest bars and spaces are resolvable where the beam spot has the
smallest cross sectional dimensions. This narrow region is sometimes
referred to as the beam waist.
In normal practice the range of distance over which the most dense codes
may be resolved with a fixed focusing lens is quite short. For example bar
code targets with 5 mil, (0.005 inch), wide bars and spaces may only be
resolvable over a range of 1 or 2 inches when the waist is located at
about 6 inches from the laser source. Such readers are perceived to have a
sensitive "sweet spot" when attempting to read dense codes.
Beyond the beam waist where the beam diverges it is only possible to
resolve wider bars and spaces. Thus 10 mil bars and spaces can be resolved
farther out and over a somewhat greater range than 5 mil ones. Likewise 20
mil wide bars and spaces can be resolved over a greater range than those
having 10 mil widths however it is very desirable for portable equipment
to have a longer depth of operating field for the dense 5 and 7 mil bar
codes.
In the U.S. patent application entitled "ELECTRO-OPTICAL SCANNING SYSTEM
WITH GYRATING SCAN HEAD", Ser. No. 07/776,663 of which the present
invention is a continuation-in-part, several methods were described for
extending the depth of field of a beam scanning bar code reader. One of
these methods employ the use of a moving lens system and another employs a
non-imaging cone shaped optical element to provide a beam which is narrow
over a certain distance and diverges rapidly beyond that distance.
U.S. Pat. No. 4,816,660 describes the use of a conventional aperture stop
to increase depth of field of a laser bar code reader. The aperture stop
has a draw back in that it wastes or "throws away" a substantial portion
of the laser beam power to gain increased depth of field.
Besides the depth of field problems associated with beam spot size, the
detection of reflected light signals from both close and distant targets
presents a problem regarding signal strength which is also perceived as a
working distance limitation for scanning equipment. In a bar code reader
when light is scanned across a target it is diffusely reflected over a
large solid angle and only a small portion of the reflected light is
received by its light detector. Since the amount of light received from a
target close to the scanner is much greater than that received from a more
distant target, the signals produced by the photo detector are strong for
the close targets and weak for the distant ones. In either case however,
the signals must be detected, amplified, shaped, and digitized.
To detect the weak light signals from the far out targets, the signals from
the photo detector must be amplified and the outgoing beam must be strong
enough to provide an ample signal to noise ratio for error free
information processing. Alternatively, if the target is close, to the
scanner the light returned from the target can be so strong that it
results in saturation of the signal amplifiers. In addition, strong
optical noise is received from the surface of the close target itself due
to its natural surface texture and the surface noise signals can be so
strong that they falsely trigger pulse shaping circuits to give the
perception to a user that the apparatus lacks depth of field.
In order to overcome the signal processing difficulties manifested as
undesirable wide dynamic range of the returned signal amplitude associated
with both strong and weak optical signals as well as target noise;
automatic gain control (AGC) amplifiers are usually employed to supply a
somewhat constant amplitude signal to conventional comparator circuits.
Such AGC circuits are notoriously difficult to design and optimize.
Limiter circuits may also be added to prevent amplifier saturation but
they tend to degrade signal to noise ratio.
U.S. Pat. No. 5,130,520 describes how a defocused Fresnel lens placed in
front of a photo diode can help ameliorate the problems associated with
variation in the amplitude of light signals for close in and far out
targets but this requires significant spacing between the Fresnel lens and
photo diode and adds to the bulk of the system. The same U.S. patent also
describes the use of louvers to deal with the problem of signal amplitude
variation but these also lead to bulky layout and spacing requirements
unsuitable for compact designs.
SUMMARY OF THE INVENTION
The present invention employs novel non-imaging beam conditioning optical
elements which allow nearly full power usage from a light source in a beam
scanning information readout device such as a bar code reader. At the same
time the novel optical elements disclosed herein provide multiple focal
ranges which may be made to overlap thereby significantly extending the
depth of scanning range over prior art scanners.
Besides the improvement obtained by utilizing light which would have been
otherwise wasted in the prior art scanning devices, the present invention
uses this light by apportioning a greater amount of the light to the far
out targets and less to the close in targets. Thus the present invention
effectively solves the problem of diminishing signal amplitude over a long
working range while providing a small spot size to extend optical field
depth. Amplification of light signals is therefore much more straight
foreword, requiring simpler electronic signal conditioning circuits
thereby eliminating the need for complex and fussy AGC amplifiers and
limiters to deal with both strong and weak signals from close and distant
targets respectively.
Novel signal processing circuitry is also disclosed for adaptively
processing received signals and for reducing variation associated with
laser diodes, optical elements, and target related variables such as
distance, reflectivity and contrast.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows how a converging lens creates a narrow waist region in the
beam from a laser light source.
FIG. 2 shows how an aperture stop is used to make a beam cross section
smaller while sacrificing beam power to extend the depth of field.
FIG. 3 shows an embodiment of the present invention and how it creates an
extended depth of field without sacrificing beam power.
FIG. 4 shows another embodiment of the present invention with a mounting
means.
FIG. 5 shows a more sophisticated embodiment of the present invention for
producing three focal ranges which may be designed to overlap.
FIG. 6a shows a non-imaging multi-zone optical element placed after a
converging lens for producing many focal zones.
FIG. 6b shows another non-imaging multi-zone optical element for producing
many focal zones.
FIG. 7 shows an integrated multi-zone optical element combined with a lens.
FIG. 7b shows a further integrated optical element combined with a lens in
accordance with the present invention.
FIG. 8a shows a two zone element with a slot aperture.
FIG. 8b shows a two zone element with an elliptical aperture.
FIG. 8c shows a multi-zone element with three areas for light to pass
through.
FIG. 8d shows another two zone element.
FIG. 9 shows an optical element using dual reflectors for producing
multiple virtual light sources and multiple focal ranges.
FIG. 10 shows signal diagram of how trigger thresholds are set to produce
digital signals.
FIG. 11 shows a schematic diagram of a novel adaptive hysteresis circuit
for digitizing signals.
FIG. 12 shows a schematic diagram of a circuit for controlling laser output
power as a function of received signal strength.
FIG. 13 shows a light source, collimating lens and a cone shaped
non-imaging optical element generating a long focal range.
FIG. 14 shows a two zone element of the present invention for use in a bar
code target scanner.
FIG. 15 shows an embodiment of the present invention wherein unwanted light
is scattered by a translucent area in an optic means.
DESCRIPTION OF PREFERRED EMBODIMENTS
While the following description may refer to a bar code reader and the
items being read as bar code targets, it should be understood that the
invention is applicable to other information readout devices and equipment
requiring beam conditioning as disclosed herein. Accordingly, as used
herein, the term target refers not only to a bar code but to any item
having optically discernible features which require detection and
discrimination and optical signal may represent any electronic or optical
signal of unpredictable amplitude.
As shown in FIG. 1 a light source such as a semiconductor laser 1 produces
a light beam emanating from source S, which is focused by a positive
converging lens 2.
The light from source S converges down range to an area known as the beam
waist 3 which is the area of greatest power concentration or intensity of
the beam. In the waist region the light beam has its smallest cross
section. Thus in the vicinity of the beam waist the highest resolution is
obtainable and the finest target detail is resolvable. In practice the
beam is shaped to converge and diverge gradually as depicted by beam
profile curve 7 so as to produce a useful range R of field depth for bar
code reading.
If an aperture and stop is placed in front of the converging lens as shown
in FIG. 2, the widely diverging portion of the beam typified by ray 10a is
blocked by stop 8, whereas a less divergent portion of the beam as shown
by ray 11 passes through aperture 9. The result is that ray 11 moves down
range unimpeded whereas Ray 10a which would have followed dotted path 10b
is blocked.
If the aperture stop were not used as in FIG. 1, then the useful range
where the beam size would be small enough to resolve a bar code target is
depicted by range R. Due to the aperture stop the range where the spot
size is useful will be relatively longer as indicated by R' however, light
is wasted. This of course is in accord with well known principles of
geometric optics, Gaussian beam geometry and stop aperture techniques.
Now turning to FIG. 3 we see a transparent window 14 having an aperture 15
in it. The transparent window has a thickness T and an index of refraction
N>1. This transparent aperture window is located between real light source
S and converging lens 2.
Rays emitted from source S such as ray 16a and 16b which diverge at angles
small enough to pass through aperture 15, will focus at f1 after passing
through positive lens 2.
On the other hand rays such as 17a and 18a also emitted from source S which
diverge at an angle too large to pass through aperture 15 will pass
through the window 14 and undergo refraction. These more divergent rays
emerge along paths typified by rays 17b and 18b respectively. After
passing through converging lens 2, rays such as 17b and 18b then converge
in the vicinity of f2 farther down range in accord with the principles of
geometric optics. Accordingly, if a laser light source is used the beam
will form two waist regions characteristic of Gaussian beams. Because of
refraction, the light which passes through the refractive window 14 such
as original rays 17a and 18a behave as if they had originated at two
different source points, one being a virtual source point S' which is
closer to converging lens 2 than is real source point S.
The distance X between source points S and S' in air is equal to (N-1)T/N,
where N is the index of refraction of the refractive window material and T
is its thickness. This along with the well known formulas for image
position can be used to design similar window elements with focal ranges
as desired.
It should be remarked here that the virtual source S' location is different
from the real source S location because as portions of the beam pass
through media with different indices of refraction, different optical path
lengths are created. Any number of optical paths may be created in this
manner and when rays from the various paths are passed through a
converging optical element such as a positive lens the rays from the
different sources will converge at different points down range. Since this
system has no single focal length it is inherently a non-imaging beam
conditioning device.
In a preferred embodiment as shown in FIG. 3 the aperture 15 is made small
so that only a small fraction of the light passes through it for the close
in targets and a substantially greater amount of light is allowed through
the window part of the transparent substrate 14 for focusing upon distant
targets.
Since the proportion of light returned from distant targets will be far
less than the proportion returned from the close targets, more light is
allocated to the distant ones and focused down range. The smaller portion
of light which is focused on close targets is easily detected up close but
is out of focus for the distant ones and has an insignificant effect upon
the light signal detector for these.
Alternatively, the light allocated for the distant targets is not yet in
focus to resolve close targets and hence does not produce well modulated
pulses when it sweeps across the close targets but produces sharp pulses
when swept across distant ones and is thereby detected. In this way the
dynamic range problem of varying light intensity over large distances is
solved while small spot size is provided up close as well as down range.
Digitization of the sharp high speed electronic pulses, produced with
multi-zone beam shaping elements is accomplished with special circuits
designed to be responsive to abrupt variations in reflected light signals
produced by light swept across a target. The use of differentiator
circuits and electronic filters are also employed to accomplish the
required signal discrimination.
While it is highly desirable to not waste light from the source, it is well
known that the angle of divergence of light from laser diode light sources
can vary by several degrees from one unit to another. In order to obtain
consistent results and predictable behavior from diode to diode it is
desirable to eliminate light which diverges from the source at an angle
greater than an allowable maximum. This may be accomplished by adding an
annular area 23 as seen in FIG. 4 to preclude the unwanted excessively
divergent light from reaching the target.
In a preferred embodiment (shown in FIG. 15) annular area 23 is simply
given a frosty or translucent finish for scattering unwanted light but it
may, of course, be made opaque to block the unwanted light.
The transparent aperture window 14 may be made from glass or plastic film
with the hole etched or drilled and the light scattering area 23 frosted.
Odd shaped apertures and other features may be etched into glass windows
using hydrofluoric acid and photo-lithographic techniques. In order to
center and mount the window an etched shoulder 19 may be provided to
center it on a lens having a rim 20. Such lenses are available in molded
plastic from Universe Kogaku (America.), Inc. and are used in CD players.
They are available with focal lengths of 3.3, 4.5, and 3.9 mm. A suitable
thickness for the transparent aperture window has been found to be from
0.004 to about 0.008 inches and was originally fabricated from thin glass.
The window was mounted between a laser diode and a lens having a 3.9 mm.
focal length.
This combination can be focused so a first beam waist occurs at about 10
inches away from the light source and a second at about 18 inches away
with excellent resolution near in and far out. Other combinations are
easily arranged as dictated by performance requirements and the design
formulas previously cited.
Hole sizes on the order of 0.040 inch to 0.025 inch diameter are easily
fabricated commercially and would be well suited for this system to
allocate adequate light far out. Not much light is needed or desired for
close targets.
FIGS. 8a, 8b 8c and 8d show other window element configurations having a
slot shaped opening, 50 an ellipse shaped opening, 72 a solid bar of
refractive material 74 and a semi-circular openings 76. Each of these
elements can produce useful multi-focal beam properties according to the
present invention.
The concept of dual zone focusing as explained above for the two zone
embodiment may be extended to many zone focusing. In FIG. 5 a second
transparent refractive step 21 is added to the transparent refractive
window 14 to create three distinct optical path lengths through the
device: (1) straight through clear aperture 15, (2) through clear aperture
22, then through refractive window 14, (3) through refractive layer 21,
then through refractive window 14. Light traversing these three distinctly
different paths followed by a positive lens will converge at three
distinctly different zones down range in the manner previously explained
for dual zone focusing.
When a laser light source with a Gaussian beam output is used with the
multi-zone elements as described herein, the light does not focus to
distinct points. Instead, each distinct optical path creates multiple beam
waist regions or "focal ranges" down axis. If these waist regions are
spaced appropriately they can be made to create overlapping focal zones
thus providing a beam which is narrow over a certain distance range and
which diverges beyond the last focal zone in accord with concepts
elaborated upon in U.S. patent application Ser. No. 07/776,663 of which
the present application is a continuation-in-part.
In FIG. 6a a multi-zone, non-imaging, optical element 52, according to the
present invention is illustrated which will yield four separate focal
ranges f1, f2, f3, and f4 when inserted after a focusing lens. (In this
case element 52 does not produce multiple virtual sources as do elements
in previous examples.) It should be remarked here that although optical
element 52 may look like a Fresnel lens due to its concentric zones 53,
54, 55, and 56 but it is not a Fresnel lens. Unlike a traditional Fresnel
lens, it is not an imaging type optical element because it does not posses
any particular focal length since concentric zones 53, 54, 55, 56 cause
rays 93, 94, 95, 96 to converge as rays 83, 84, 85, 86 at different foci
f1, f2, f3 and f4, respectively. The angle of each concentric zone may be
made to decrease or increase, as shown in FIG. 6a, from the center
outwardly as dictated by design requirements, the principles of geometric
optics and gaussian optics if a laser source is used. If the angles of the
concentric zones are designed to decrease from the center outwardly, then
the order of the focal ranges as seen in FIG. 6a may be reversed so that
the outermost concentric zones cause light to focus at the more distant
ranges. Instead of several discrete zones shown on element 52 of FIG. 6a,
FIG. 13 shows an element 405 which generates a continuously narrow beam
406. In the system of FIG. 13, a light source 1 generates a beam 401 which
is collimated by lens 2 to form collimated beam 404 after which it is
converged by cone-shaped non-imaging optical element 405 to generate a
continuously narrow beam 406 which remains converged along length 407,
then rapidly diverges at point 408 finally dispersing in area 409.
In FIG. 6b, an element 32 is shown having concentric zones 33, 34, 35, and
36, which may be placed between a real source and a converging lens. Light
emitted from the real source will appear to emanate from multiple virtual
sources, and after passing through an appropriate lens will converge at
multiple focal distances down range. The angled zones can be designed to
have a kind of positive or negative optical power so that such elements
may be placed in front of a converging lens and each zone will cause light
to converge at a different place down range. These zones can also be sized
to apportion the desired amount of light to successive focal zones.
In preferred embodiments the circular areas of each step of the multi-zone
elements described herein are sized to focus an increasingly greater
amount of light on the more distant targets in order to compensate for the
signal amplitude variation due to target distance losses as previously
described. Typically a good first approximation to this design would be to
apply a square law relationship for range versus the proportion of light
allotted for the various zones; i.e. as distance from the light detector
to a focal zone is doubled the amount of light allotted to the zone is
quadrupled.
A preferred fabrication method is to make the multi-zone element from
injection moldable plastic. In yet another embodiment the multi-zone
element would have its zonal features molded into a single integrated
optical element as illustrated in FIG. 7 with stepped zones 44, 45, 46,
and 47 molded into element 41 and a lens surface 40 all combined as a
single part. Also the area of each zone may be sized to cause increasing
amounts of light to be focused farther down range.
It is also anticipated that the multi-zone element as described herein
could be fabricated in mass volume as a holographic optical element (HOE)
as was one of the beam shaping elements described in the U.S. patent
application of which the present application is a continuation-in-part.
As shown in FIG. 7b, the multi-zone, range extending, optical element of
the present invention can be designed with a specially contoured
non-stepped surface shape to effect continuous extended zone focusing and
beam power characteristics down range. In order to generate the required
characteristic surface curvature, the element is first designed with many
discrete zones. Then using standard mathematical curve fitting techniques
the equation of curvature is empirically derived. Numerically programmed
diamond machining techniques are then applied to produce the required mold
cavities.
Another embodiment of the present invention is shown in FIG. 9 wherein
transparent substrate 67 has mirrored surfaces 60 and 61 and is located
between primary light source S and positive lens 2. A small aperture 65 is
provided in reflective surface 60 to allow light from source S to pass
through. On the opposite side of substrate 67 is a partially reflective,
partially transmissive surface 61. As light rays from source S pass
through aperture 65 a portion of them such as rays 62 and 64 will continue
through partially reflective coating 61 as typified by rays 66, 68 and 70
proceed through converging element 2, whereupon they converge down range.
A substantial portion of the light from source S encountering partial
mirror 61 will, however, be reflected back from surface 61 toward surface
60 then again toward surface 61 as typified by ray 62. If the path of ray
62 is traced back toward source S it will seem to come from a virtual
source different than S. Thus rays such as 62, after passing through lens
2, will converge down range at a different place than non-reflected rays
and the multiple reflections set up between the two mirror surfaces 60 and
61 will give rise to multiple virtual sources and the desired multiple
focal ranges.
SIGNAL PROCESSING CIRCUITS FOR USE WITH THE OPTICAL ELEMENTS
Not only are the beam characteristics such as focal lengths and power
distribution important in determining the overall useful operating range
of a beam scanner but so is the signal processing circuitry. As previously
pointed out the strength of the returned light signal and its noise
content must be considered when processing the information signal into a
computer readable form. The various embodiments of the optical element
previously described address the proportion of light allotted to close and
distant targets as well as spot size and range but other factors also need
consideration. For example, overall light output of the light source
proportionately affects the amount of light received by the light
detector. This can vary with operating conditions such as temperature and
age of the light source. Also target properties such as surface texture,
color, and reflectivity can have a major influence on the amount of light
returned to the photo detector. To a user these factors will affect the
perceived goodness of depth of operating range. To effectively deal with
these effects an adaptive trigger circuit has been invented and is now
described.
ADAPTIVE TRIGGER CIRCUITRY
The novel adaptive circuit may be understood by turning to the schematic
diagram shown in FIG. 11. As light signals 100 from a target enter photo
detection diode 102 they are converted by amplifier U1 into analog signals
representative of target features which are passed to differentiator
circuit 104 comprised of R1,C1,R2,C2,U2 the output of which is made
primarily responsive only to high speed abrupt changes in the signal
resulting from scanning focused light across a target and not to unwanted
low frequency signals. In a bar code reading application these abrupt
changes correspond to movement of the light beam as it makes a transition
from a bar to a space or vice versa. It is desired to convert the
differentiated waveform into square wave pulses of constant height having
widths proportional to the time taken for the scanning spot to pass the
bars and spaces of the code in order to decode them.
FIG. 10 illustrates the signal processing required for two differentiated
waveforms of differing signal amplitudes and noise levels. Typical
information signal peaks such as 200, 201 and 230, 231 swing above and
below a common reference voltage, Vref. The shorter peaks 200 and 201 may
come from a distant or poorly reflective target and are accompanied by
background noise peaks 204, 205 while strong peaks 230, 231 may come from
a close target and are accompanied by proportionately stronger target
noise peaks 224, 226. In order to square up weaker signals 200, 201 a
comparator circuit with hysteresis may be employed which is set to trigger
ON above the maximum noise peak 204 when the signal amplitude reaches
trigger threshold 210 indicated by a dashed line and OFF when the signal
waveform drops to a low threshold level 212 below noise peak 205. In this
way a corresponding square wave 214 may be generated which has a time
width 216 approximately proportional to a bar or space as the case may be.
Unfortunately if the signal were a strong signal such as that represented
by peaks 230 and 231, the corresponding noise level peaking at points 224
and 226 would falsely trigger the comparator at preset trigger levels 210
and 212.
For the stronger signals it is necessary to trigger ON and OFF at different
levels 220 and 222 to prevent false noise triggering and to generate
square pulses such as 236 with amplitude 244. These different levels
however would fail to trigger on the weak signals 200, 201 produced by
distant targets. One would want to set the trigger levels automatically so
they are appropriate what ever the signal amplitudes are while avoiding
false triggering by the noise. The adaptive trigger circuit of FIG. 11 has
therefore been invented to do just this.
The differentiated waveform output of U2 is fed into a high speed positive
peak detector 106 which outputs a DC voltage equal to the peak
differentiated signal amplitude received from a target. In a bar code
scanner this peak level need not be sensed instantly but rather may be
established after a few scans since many hundreds of scans are normally
executed per second in such readers.
The output of peak detector 106 is buffered by amplifier U3 having a gain
of +1 so it may supply DC current at the positive peak differentiated
signal voltage without disrupting the output of U3. The positive peak
voltage is also fed to a (-1) inverting buffer U4 which outputs a voltage
approximately equal to the most negative swinging differentiated signal
amplitude with respect to Vref. Vref is typically chosen to be 1/2 the
voltage of supply voltage +Vcc with respect to supply ground 120.
Circuit element U5 is a high speed CMOS comparator. Texas Instruments, Inc.
makes these comparators with two units in one package. One such model,
TLC352C, can operate with a single ended supply voltage as low as +1.4
volts, and since it has an open drain CMOS transistor output it can switch
its output clear down to its negative supply rail Vee. (Comparators with
open collector bipolar transistor outputs can not do this.) In the circuit
of FIG. 9 the supply voltage +Vcc is typically about +5 volts.
Notice that the output of inverting amplifier U4 which represents a
negative signal level with respect to Vref may be small or large and is
connected to the negative power supply pin of U5 to supply a negative
power supply voltage to U5 having a value approximately equal to the most
negative signal pulse received by peak detector 106. Thus this voltage is
a variable pseudo-supply ground for U5. (It is not common to bias a
comparator with a negative peak signal voltage level used as a
pseudo-supply ground.) Normally the negative power supply pin of a
comparator is connected to supply ground and may not be quickly responsive
to changes in a non-fixed ground level such as the pseudo-supply ground
used here, but in this case it need not respond very quickly since it will
have numerous scans over which to stabilize. Notice also that even if the
negative supply voltage of U5 were to equal Vref when no signal is
received, the comparator will operate because +Vcc (at about +5 volts) is
enough above Vref (at +3 volts) to power the comparator which operates
with only a +1.4 volt supply.
Since the output of U5 can switch down to its negative supply rail its
output will drop to -V peak when its output is in the OFF state. The open
drain output of U5 is connected through pull up resistor R5 to +V peak
which represents the maximum positive excursion of the differentiated
signal amplitude supplied by the output of positive peak detector buffer
amplifier U3. If comparator U5 is triggered ON its output will swing to
the peak positive value of the received differentiated signal. Thus
comparator U5's output will go to the value of the positive peak signal
supplied by U2 when ON and to the negative peak value of this signal
voltage when OFF. The negative input of U5 is connected to Vref and U5 is
also provided with a hysteresis trigger circuit at its positive input
established by resistors R3 and R4. This hysteresis trigger arrangement
will cause the upper and lower trigger thresholds of U5 to be equally
placed above and below Vref. Furthermore, unlike prior art trigger
circuits with fixed trigger thresholds, U5 will now turn ON when the
signal voltage reaches a value (+V peak)(R3/R4) and OFF when the signal
voltage falls to (-V peak)(R3/R4) where +V peak and -V peak are the
maximum and minimum values of the signal levels respectively.
For the self adjusting trigger circuit of FIG. 11 it has been found that a
ratio of about 0.7 for R3/R4 is well suited to avoid noise levels in bar
code reading applications. Thus the circuit triggers adaptively always at
+/-70% of the differentiated signal peaks no matter what amplitude the
signal peaks may have.
The output of U5 will now produce very clean square wave transitions
symmetrically above and below Vref representative of the widths of bars
and spaces, however the output amplitude of U5 is equal to the peak to
peak amplitude of the differentiated signal which may vary from only a few
tens of milivolts to a few volts. Such a signal is not yet suited for
logic level signal processing. It must be made to swing from supply ground
up to a minimum value required by digital switching circuitry.
To achieve the final signal form needed, the output of U5 is fed directly
into the positive input of a second comparator U6 which can also be a CMOS
comparator but it may be a common bipolar type. Since comparator U5 is
supplied in packages containing two such units it is preferred that U6 be
the second CMOS comparator packaged with U5.
The negative input of comparator U6 is again referenced to Vref, but its
positive input needs little or no hysteresis since the signal supplied to
it by U5 is already a clean square wave.
The output of U6 will now follow that of U5 time wise but will be +Vcc (+5
V) when ON and -V peak when OFF regardless of the amplitude of the signal
supplied to its input.
Circuit elements C3 and D1 reference the square wave output of U6 appearing
at point 122, to supply ground 120. Junction 124 will now switch from
supply ground to about 1/2 (+Vcc). Finally the signal at junction 124
drives high speed switching transistor Q1 through resistor R7 whereupon a
clean square wave output appears at output terminal 130 which goes from
supply ground to +Vcc in accord with received signals having a wide range
of signal amplitudes.
This adaptive hysteresis trigger circuit generates square wave transitions
at output 130 required by digital signal processing equipment regardless
of incoming signal amplitude. Few parts are needed and the design is
robust, requiring only that the ratio R3/R4 be selected. In addition the
circuit complements the beam shaping methods described earlier to deliver
a high level of performance with respect to depth of range in beam
scanning equipment, such as bar code readers, object and edge detection
devices and the like. Furthermore it is contemplated that this novel high
speed adaptive hysteresis trigger circuit may also be used to shape and
condition signals in fiber optic receivers, optical local area network
receivers, RF information receiving equipment and other information signal
processing equipment.
LIGHT SOURCE CONTROL CIRCUIT
In certain cases, especially where targets are to be read at close ranges,
the full power of a laser diode light source can be so strong that it will
generate substantial optical noise when swept across a textured surface.
Also the life of laser diodes can be shortened by operating them at high
power. The circuit described below solves these problems while providing
the higher power output levels when required to read distant targets. In
addition to these benefits, the circuit shown in FIG. 12 provides a
readily processable signal for both near and far targets.
The amplified photo detector signal such as that derived from the output of
the differentiator circuit 104 of FIG. 11 is fed into the input 302 of
buffered signal peak detector 304. A proportion of the buffered peak
detector output signal is input to the negative input of amplifier A1
through R10. Amplifier A1's output drives the base of transistor Q2 which
controls the current supplied to laser diode LD and therefore its light
output. If the received signal is too strong transistor Q2 thereby reduces
the light output of the laser diode and increases the light level if the
received signal is too weak.
Current drawn by laser diode LD is sensed across load resistor RL and a
portion of this signal is returned to the negative input of A1 through
R11. The value of RL is typically chosen to be a few tens of ohms and R11
and zener reference voltage Vz are chosen to limit the current supplied to
laser diode LD to a safe level to prevent damage to it.
The circuit will control the output of the of a laser diode to deliver
adequate optical power to the target while maintaining returned signals at
conveniently processable levels while extending the service life of the
laser.
While particular embodiments of the present invention have been illustrated
and described herein, it is not intended to limit the invention and
changes may be made therein and still remain within the spirit of the
following claims.
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